As the parts for outer panels of cars and household electric appliances and the like, shaped articles of polyolefin of which constituent unit is an olefin such as ethylene, propylene and the like are widely used in place of metals. These shaped articles are frequently coated with two-package type top paint containing polyisocyanate compound, and in such occasions, usually chlorinated polyolefin-containing primer is applied in advance, for improving adherability between the top coating film and the shaped articles.
Conventionally, a rubber component (e.g., styrene butadiene rubber, isoprene rubber and the like) or hydroxyl group-containing polyolefin was added to polyolefin shaped articles, for improving adherability or impact resistance. In recent years, however, addition of such a component has been quantitatively reduced or entirely stopped, which gave rise to a problem of deterioration in adherability of conventional primer with such shaped articles.
With the view to solve this problem, the present applicant previously proposed a primer which used specific chlorinated polyolefin and blocked polyisocyanate concurrently with polyol resin (cf. JP 2002-121462A). For this primer, aromatic organic solvent such as toluene or xylene is used to secure solubility of the chlorinated polyolefin, but conversion to water-borne compositions is required also for primer from the viewpoints of sanitary safety and environmental preservation.
Various attempts are made to prepare aqueous dispersions of chlorinated polyolefin, to meet the above demand. For example, JP Hei 3(1991)-182534A and JP Hei 6(1994)-256592A disclose preparation of aqueous dispersions of chlorinated polyolefin comprising converting unsaturated carboxylic acid- or acid anhydride-modified chlorinated polyolefins to water-based compositions, using emulsifier and basic substance, and further using as an emulsifier a compound having polyoxyalkylene structure. The methods disclosed in these patent publications, however, require use of a large quantity of the emulsifier for dispersing the polyolefin in water, and primers in which the resulting aqueous dispersions are blended may form coating film having insufficient water resistance, humidity resistance and gasohol resistance. In particular, when the coating film is thick and baked at such low temperatures as not higher than 90° C., water resistance, humidity resistance and gasohol resistance of the coating film are apt to decrease.